Eccentric hamstring strength in young athletes is best documented when normalised to body mass: A cross-sectional study with normative data of 590 athletes from different age categories

Despite its widespread use in adults, the Nordic hamstring exercise remains underexplored in athletic youth populations. Further, given the dynamic nature of growth and maturation, comparisons with elite adult populations may be inaccurate. Here we describe absolute and body mass-normalised eccentric hamstring normative values for football, athletics and multi-sport youth populations. 676 routine standardised tests were conducted, including 244 U12–U18 student-athletes (football, athletics, multi-sport) and 346 Qatar Stars League (QSL) football players using the NordBord. The average maximum values for the left and right leg from 3 repetitions were recorded. Significant increases in absolute strength were seen across chronological (e.g., 150 N ± 15 for U12 to 330 N ± 40 for U18) and skeletal (142.9 N ± 13.9 for skeletal age of 12 compared to 336.2 N ± 71.2 for skeletal age of 18) age groups. The differences in values normalised to body mass were smaller at 3.6 N/kg ± 0.25 for the U-13 group, but similar for the U14 to U18 groups (4.5 N/kg ± 0.16, 4.6 N/kg ± 0.11, 4.6 N/kg ± 0.27, 4.7 N/kg ± 0.14, 4.5 N/kg ± 0.18). Students displayed lower absolute strength than the professional football players (272.1 N compared to 297.3 N, p < 0.0001) but higher relative strength (4.7 N/kg compared to 4.2 N/kg, p < 0.0001). Comparing Nordic hamstring strength values between athletes, and between skeletal and chronological age groups can be done when values are normalised to the athlete’s body mass. For the U14s and onwards age categories, body mass normalised values are comparable to professional football players.

sporting academy in the Doha, Qatar.Testing was completed during the preseason or initial competitive cycle of the 2016/17, 2017/18 and 2018/19 seasons (Figure 1).A total of 330 male athletes (chronological mean age 15.3 ± 1.7, height 169 ± 11, weight 58.8 ± 14.2, BMI 20.3 ± 3.3) presented for screening across three seasons from 2016 to 2019.The demographic data for this cohort are summarised in Table 1.There were 313 body mass measurements and 306 complete skeletal age measures conducted at the time of eccentric hamstring testing assessments.All athletes needed to be free of injury to the lower limbs and able to participate fully in training at the time of testing.

Qatar Stars League footballers
The comparison adult data are drawn from a previously published study [4] which prospectively examined professional footballers in the Qatar Stars' League over a 4-year period during annual precompetition medical assessment.In year 2 of this study, all eligible players were tested on the NordBord Nordic hamstring testing device.
After exclusion of the players who were injured or refused to consent to the testing, 346 players' results were available for this analysis.

Data collection procedures and statistics Eccentric knee flexor strength
Athletes performed one set of three maximal repetitions on a device specifically designed to measure maximal force output (N) during the Nordic hamstring exercise [5] using previously described methods.
yet to be fully established.Further, a comprehensive overview assessing eccentric hamstring strength between elite footballers and elite youth athletes is yet to be explored.The inherent complexity and non-linearity of youth athletes' normal physical growth, biological maturation, and behavioural development make it difficult to gauge a true representation of their abilities at any given moment [3].
These complex interactions need to be considered when assessing eccentric hamstring strength at a single time point, or when comparing different measures over time.To better understand the effectiveness of training interventions, any improvements in strength which are attributable to these training effects need to be teased out from those due to physical growth and maturation.[1,2] This paper aims to describe normative values of eccentric hamstring strength in well-trained male athletic and football youth populations.Secondly, it aims to describe any relationships between eccentric hamstring strength and body mass, skeletal maturation status, and chronological age.Finally, these data are compared with a sample of professional adult male football players.

Study design and participants Student athletes
A cross-sectional cohort study design described growth, maturation, and knee flexor strength prospectively over three seasons.The participants were male full-time student athletes, enrolled in the football, athletics or multi-sports programmes at Aspire Academy, an elite FIG. 1. Flow chart demonstrating the movement of players and repeated measurements during three seasons, 2016-17 to 2018-19 and body mass (kg) for highly trained athletes in sports academy.TABLE 1. Participant characteristics.Student athletes compete in age categories "U13" to "U20" -Under 13 years of age at the start of the academic year to under 20 years of age respectively.Of these student athletes, 174 played football, 119 competed in athletics, and 46 in multi-sport events.The QSL Adult cohort is the professional football players included in pre-competition medical assessment.Data are presented as mean ± SD.Skeletal age is assessed using the FELS method.No skeletal age data are available for the adult QSL athletes.

Age Group
Chronological Age (Years)

Skeletal bone age and anthropometry
Skeletal maturation was assessed at the beginning of each academic year, using X-ray images of the athlete's left hand and wrist complex taken at the Radiology Department at Aspetar Orthopaedic and Sports Medicine Hospital.The images were interpreted and entered into an academy maturation database by the same experienced assessor.Skeletal age was determined using the Fels method, and were uploaded to a central academy anthropometry database, following the procedures previously described [7].
Nordic strength compared to chronological and skeletal age

DISCUSSION
This cross-sectional study of highly trained male youth athletes showed a clear pattern of increasing strength with chronological and skeletal age.However, the more important clinical finding is the more consistent relationship between body mass and maximum eccentric Nordic strength across these chronological and skeletal ages, particularly once these athletes were above the age of 13.This information can be used by physical preparation and rehabilitation practitioners as benchmarks for their adolescent male cohorts independent of their size and age.While this was a cross-sectional study, the stability of this finding across ages suggests that longitudinal progress needs to be assessed using the body mass-normalised values lest the apparent maturation effect be mistaken for relative improvement in strength.
These data suggest that clinicians should consider body mass as a critical factor in the interpretation of absolute eccentric hamstring strength.Increased body mass and/or longer lower leg lever distances can influence eccentric hamstring strength results during the Nordic exercise [8].Specifically, heavier and older players have been shown to outperform their younger, lighter counterparts [8].Body mass was found to be largely responsible for observed age-related increases in absolute Nordic hamstring strength [9].A large body of evidence suggests that performing the Nordic hamstring exercise as part of a prevention programme is an effective way to reduce this injury's burden [1].

Data analysis
Absolute force data for the left and right limbs were entered in a spreadsheet (Microsoft Excel Microsoft, Redmond, USA) which calculated the average of both legs for the repetition where the highest force was displayed for each leg during the three contractions -the "mean peak force" (average of both limbs' highest score).This value was recorded in both absolute terms (N) and relative to body mass (N/kg).Statistical analysis was conducted in JMP (JMP, Version

Ethical approval and consent
This study was part of a larger study on growth, maturation and athletic development for which written informed consent was obtained from the athletes' guardians prior to data collection and ethics approval was granted from the Anti-Doping Lab Qatar Institutional Review Board (IRB Application #E20140000012).The adult data were collected as part of the routine pre-participation periodic health evaluation for football players participating in the Qatar Stars' League and ethical approval for this cohort was obtained from the Shafallah Medical Genetics Centre (institutional review board project number 2012-020).

Student athlete Nordic hamstring strength -absolute and relative values for different age group categories and sports
These analyses were conducted for both absolute and relative Nordic hamstring strength.For absolute strength, a significant effect of age group was found (p < 0.001) and a non-significant effect of sport (p > 0.20).The interaction effect (sport and age group) was significant (p < 0.01).For the Nordic hamstring strength normalised to body mass, there was a significant effect of both chronological age (p = 0.004) and an interaction effect of sport and chronological age (p = 0.0014) whereas the effect of sport was not significant (p = 0.7112).Post hoc testing however revealed that of the possible 210 pair-wise comparisons (age group and sport), 21 were statistically significant after adjusting for multiple comparison (Tukey's HSD), Appendix 5, Bourne et al. [11] reported eccentric knee flexor strength in elite rugby union players of 3.65 ± 0.71 N/kg [11].Further, they found subelite and U19 players to be significantly stronger than elite players, which is consistent with our cohort.Again, in uninjured elite Australian footballers, in-season relative eccentric hamstring strength scores were found to be 4.09 ± 1.01 N/kg [12].

Limitations and future research
Given the limited understanding of eccentric hamstring strength in youth sporting populations, further exploration is warranted particularly in other cohorts including youth female athletes and other ethnicities.In this cohort, the lever arm was not measured and may represent an extra independent variable to consider in youth athletes.

FIG. 2 .
FIG. 2.Nordic hamstring exercise being performed on the NordBord and body mass (kg) for highly trained athletes in sports academy.
following the procedures outlined by Roche et al. [6]; here a maximal skeletal age of 18.0 indicates full maturity.Anthropometric screenings were conducted by ISAK (International Society for the Advancement of Kinanthropometry) Level 2 certified academy staff at the start and end of each season, which corresponded to the academic year.Measures were taken early in the morning prior to any activities to minimize diurnal variations, following ISAK-recommended procedures,

Figure 3
Figure 3 and Table 2 show the absolute and relative values of Nordic

16. 0
SAS Institute Inc.,Cary, NC, 1989-2019).Mixed model regression analysis was conducted considering the fixed effects of sport (Football, Athletics, and Multi-sport) with chronological age group (U13 to U18), and the interaction effects of these two factors for the student athletes (considered as random effects), with post hoc adjustment for multiple comparison (Tukey's HSD).

Figure 4 .
Post hoc multiple comparison of the different student athlete age group categories for the relative Nordic hamstring strength revealed significant differences only for the comparison between the U13 years age group and the 15 years (difference = -1.0N/kg [-1.7 to -0.2] p = 0.005), and the U13 to the U17 years age groups (difference = -1.1 N/kg [-1.9 to -0.3] p = 0.002).For each of these models, the residuals were analysed for normality by a combination of inspection of frequency histograms and residual quantile-quantile plots, and Shapiro-Wilk tests.associations (r = 0.47) with maximum force in youth.In this study, relative maximum force was found to be 4.4 N/kg across all players.

TABLE 2 .FIG. 3 .FIG. 4 .
FIG. 3. Relative (N/kg) and absolute strength values for the different age groups.Note that the "QSL" category indicates all the Qatar Stars' League players, and the U13 to U20 categories indicate the Aspire Academy student athletes' age group categories.Confidence diamond within the boxplot describes the 95% confidence limit of the mean, contours represent the distribution of the individual points (dots) for each observation.Dotted line connects the mean values of the groups.

FIG. 5 .
FIG.5.Absolute (N, upper panel) and relative (N/kg, lower panel) average maximum Nordic hamstring strength compared to age for both the student athletes (red diamonds, red line of best fit with confidence interval) and the QSL football players (blue circles, blue line).There is a moderate positive (r = 0.64 [0.57 to 0.70]) significant (p < 0.0001) correlation of absolute strength and age for the student athletes but not for the QSL footballers (r = 0.06 [-0.17 to 0.04], p = 0.2365).For the relative strengths, there is a weak positive (r = 0.25 [0.14 to 0.36]) significant (p < 0.0001) correlation for the student athletes, and a weak negative (r = -0.21[-0.31 to -0.11)] significant (p < 0.0001) correlation for the QSL football players.

Table 2
The Nordic hamstring exercise requires a level of technical competence and a capacity to tolerate a high loading stimulus, which limits these findings to athletes who have been given appropriate technical instruction.The cross-sectional nature of this study prevents definitive statements about the longitudinal stability of these measures during maturation irrespective of the apparent stability of these data.Accordingly, long-term data examining individual variation are suggested for future research.Finally, while the performance of the APPENDIX 1.All pairwise comparisons of the different age groups for the absolute values of Nordic hamstring strength showing the mean, standard error of the difference, and the lower and upper 95% confidence limits.Significant (p < 0.05) differences are highlighted in red font (post hoc adjustment for multiple comparison using Tukey's HSD) All pairwise comparisons of the different age groups for the relative values of Nordic hamstring strength showing the mean, standard error of the difference, and the lower and upper 95% confidence limits.Significant (p < 0.05) differences are highlighted in red font (post hoc adjustment for multiple comparison using Tukey's HSD) All pairwise comparisons for the different skeletal age groups for the student athletes for absolute Nordic hamstring strength.Significant differences are noted in the column "Prob > |t|" with an asterisk and a coloured font.All pairwise comparisons of relative Nordic hamstring strength for the different skeletal age groups.No significant differences were found for any between-group comparisons.All pairwise comparisons of relative Nordic hamstring strength for the student athletes -age and sport, with post-hoc adjustment (Tukey HSD) for multiple comparison.Significant effects are highlighted in the Prob > |t| column with an asterisk and coloured font.